Photoconductive layers and process for electrophotography



y 1966 G. SCHAUM ETAI 3,253,914

PHOTOCONDUCTIVE LAYERS AND PROCESS FOR ELECTROPHOTOGRAPHY Filed NOV. 2, 1962 10- FIG. 7

INVENTORS.

GUS 7I4V SCHAUM, H/LDEGARD HAYDN. Alg TA VON KN/G,

HEINRICH SEIBERT. a F

ATTO PNE Ys United States Patent 15 Claims. (Cl. 96--1) This application is a continuation-in-part of our application Serial No. 696,032, filed November 13, 1957, now

abandoned.

The present invention relates to photoconductive layers for electrophotography.

For the production of photoconductive layers for electrophotography it is known to use certain inorganic or organic photoconductive compounds. Examples of such compounds are sulphur, selenium, oxides, sulphides and selenides of zinc, cadmium, mercury, antimony, bismuth, and lead, and also anthracene and anthraquinone. These substances are applied to an electrophotographic plate, especially a plate consisting of metal or another material, for example paper, or to a sheet or foil produced from a film-forming plastic. If necessary, the photoconductive substance can be held dispersed in an electrically isolating, layer-forming binder. Such layers being produced by means of coating solutions in which the binders are either dissolved or dispersed and in which the electro-conduetive substances are dispersed. Such photoconductive layers are for instance disclosed in United States Patent No. 2,297,691; 2,357,809; 2,727,807; 2,727,808; 2,735,784; and 2,735,785, and in French Patent No. 1,113,933; 1,122,275; 1,125,235; and 1,136,- 146.

It has now been found that electrophotographie layers with very good properties are obtained if compounds of the following formula are used in such electrophotographic layers:

These compounds are condensation products of aromatic aldehydes with heterocyclic substances having at least 1 active methyl group.

In the formula Y represents methylidene or carbon radicals required to complete a heterocyelic six membered heterocyclic ring to which a benzene or naphthalene ring is fused, any of which rings may be further substituted by a radical of the group consisting of alkyl, aryl, amino, alkylamino, arylamino, halogen, nitro, alkoxy, and aryloxy, X is a radical of the group consisting of the halogenic, sulfo, p-toluene-sulfoand perchlorato, R represents one or more substituents of the benzene ring from the group consisting of alkyl, aryl, halogen, nit-r0, amino, hydroxy, alkylamino, arylamino, alkoxy and aryloxy, and R represents an alkyl or an alkyl-substituted aryl radical, and n=0 or 1 m=l, 2 or 3.

These compounds may be applied in the cis or trans form (compare Journ. Chem. Soc. 77, pages 1687-90 (1955)).

Examples of such compounds are:

"ice

\N/ -o H=C n-Q-or I CH3 0503011 Cl O N no=rio N01 VII VHI

on oso on OCH;

XII

XIII

oh, osoaom The photoconductive compounds according to the invention are advantageous over prior art because they are more sensitive to longer wavelength. It is furthermore possible to sensitize these substances for still longer wavelengths by suitable compounds as they are for instance disclosed in French Patent No. 1,125,235.

The coating solution which is used for the production of the photoconductive layers comprises at least one of the aforementioned substances and a film-forming binding agent, the electrical specific resistance of which must be higher than that of the photoconductive substance and also higher than that of the layer support. The best results are obtained with binding agents the electrical specific resistance of which amounts to at least 10 ohms per centimeter.

Particularlysuitable binding agents are silicone resins such as alkylpolysiloxanes and arylpolysiloxanes, especially phenyl and methyl polysiloxanes as they are dis closed in German Patents No. 853,351 and 865,975.

Further suitable binding agents are for instance cellulose, cellulose esters, cellulose ethers, polyvinyl, chloride, polyurethanes, polyesters, polyamides, polycarbonates with a base of di-(monohydroxyaryl)-alkanes, especially 4,4-di(monohydroxyaryl)-alkanes according to German Patent No. 971,790 and 971,777. Photoconductive compounds can be present in the electrophotographic layers, homogeneously in dissolved form or heterogeneously in a suspended form whereby the particle size of the suspended pohtoconductor should be as small as possible.

The quantitative ratios between the photoconductive substances and the binding agents may vary within wide limits. It is preferred to use the photoconductive substance in amounts between 1 part for each 0.3 to 2 parts by weight of binding agent which is the equivalent of 0.5 to 3.33 parts by wieght of the photoconductive substance to each part by weight of the binding agent, and amounts between about 5 and about 40 grams per square meter of photoconductive layer. The solvent or solvent mixture used for the production of electrophotographic layers containing a photoconductive compound, heterogeneously dispersed therein must be good solvents for the binding agent but non-solvents for poor solvents for the photoconductive compounds. Suitable layers of this type can be produced as follows: An organic solvent is used which dissolves both the photoconductive substance and the binding agent layer former. This solution has added thereto another organic solvent in which the layer former is soluble but the photoconductive substance is insoluble.

By this means, the photoconductive compound.

' in the ball mill.

is deposited in a state of particularly fine distribution, so that layers with a particularly smooth surface are obtained.

As a support for the photoconductive layer there may be used paper or metal plates, such as zinc, aluminium, or'brass plates. Furthermore, thin foils of cellulose hydrate, cellulose esters or of polyamide come into question. In processing of the electrophotographic materials said material comprising a support of low electrical resistance and a photoconducting insulating layer, coated thereon, is electrostatically charged in the dark, for instance, by meaning of a corona discharge device. The charged layer is then exposed with light through a negative photographic film, positive film or mask or otherwise. to a light image, to receive a latent electrostatic image. If the electrophotographic material has been properly prepared, the charges leak off rapidly to the support in proportion to the intensity of light to which any given area is exposed. After such exposure, the latent electrostatic image can be developed by any developing process known per se, for example, by dusting a developing powder whereby the powder adheres to the areas where the electrostating charges remain, forming thereby a powder image corresponding to the electrostatic image. Thereafter the powder image can be fixed by melting a developing powder or can be transferred to a sheet'of transfer material resulting in a positive or negative print as the case may be.

The s-uch light-conductive layers can also be produced from aqueous dispersions of photoconductive substances and binding agents, for instance by the process disclosed in French Patent No. 1,136,146. If necessary, the cast dispersions or emulsions are subjected to a final condensation or final polymerization by heat treatment at temperatures of about -150 C. for about 2 to 30 minutes after being dried. Aqueous dispersions of polymeric substances, such as melamine-formaldehyde or urea-formaldehyde resins, xylene-formaldehyde resins, polymers based on vinyl chloride, vinylidene chloride, vinyl ethers, acrylic esters, methacrylic esters, acrylic amides, such 'for example as methacrylic amide, aromatic vinyl compounds, such for example as styrene, isoolefins such for examples as isobutylene, copolymers based on the aforementioned compounds, such for example as copolymers of vinyl chloride and butyl acrylate vinylidene chloride and butyl acrylate synthetic elastomers, such for example as copolymers of butadiene and styrene, butadiene and acrylonitrile, and also copolymers of dienes with a preponderant proportion of styrene and/or acrylonitrile or other vinyl compounds, as well as polyamides, polyurethanes, polyesters of polycarboxylic acids and polyhydric alcohols, poly carbanates, cellulose ester, rubber and the like, are for example employed.

The developing method is not especially critical and is selected according to the requirements of the special reproduction process in which the developer is to be used. Suitable developing methods are, for example, described in U.S. Patent Nos. 2,297,691, 2,573,881 and in publications of J. H. Dessauer et al. (1955) Photographic Engineering, volume 6, pages 255-261 and Metcalfe Journal of Oil and Color Chemists Association 39 (1956), page 852.

Example 1 20 g. of the compound II are added to a solution of 2.5 g. of copolymer of 30 parts of acrylonitrile and 70 parts of methyl methacrylate in cc. of dimethylformamide. The mixture is treated for several hours The slightly yellow mixture is coated on a metal or paper support and dried. vThe photoconductive layer obtained is sensitive to the visible part of the spectrum.

Example 2 30 g. of compound I are added to a mixture of 100 cc. of a 30% aqueous dispersion of a heat-hardenable Example 3 64 g. of a 60% solution of a silicone resin in toluene, for instance a silicone resin according to Example 1 of German Patent No. 853,351, 97 cc. toluene and 20 g. compound XIII are thoroughly mixed and ground for several hours in a ball mill. The resulting mixture is then cast on a paper support and dried.

The electrophotographic material can be processed according to known methods. The sensitivity lies in the visible range of the spectrum.

Similar results are obtained if in the above example the photoconductive compound XIII is replaced by, for example, compounds III, VI or VIII.

Example 4 In this example the electrophotographic properties of several compounds according to the invention are compared with known photoconductive compounds. The sensitivity of an electrophotographic material depends essentially on the difference between the photoconductivity and the dark-conductivity. The more electrostatic charge drains off by a given exposure the higher shall be the contrast respectively the quality of the resulting image. The graphs attached thereto present the draining ofi of the charge by exposure as a function of time. The steeper is the slope of the curve the more suitable is the compound for electrophotographic purposes. The axis of abscissa shows the relative charge per unit area in logarhythmic scale in arbitrary units and the axis of ordinate the time in arbitrary units.

The electrophotographic material to be tested where prepared according to Example 2 by mixing each 40 g. of photoconductive compounds with 64 g. of a solvent of the silicone resin described in that example. With each compound the processing, charging and exposure used identical technic.

The curves in FIGURES l and 2 represent the following photoconductive compounds:

We claim: 1

1. An electrophotographic material comprising a photoconducting insulating layer carried on a support, said photoconducting layer being composed of a dielectric film forming organic resin and a photoconductive organic compound dispersed therein, said photoconductive organic compound having the formula:

l 11/ R m wherein Y represents the radicals required to complete a ring system selected from the group consisting of (a) a six-membered heterocyclic ring, (b) a six-membered heterocyclic ring to which is fused a benzene or naphthalene ring, and (c) a six-membered heterocyclic ring to which are fused a benzene or naphthalene ring containing at least one substituent selected from the group consisting of alkyl, aryl, amino, alkylamino, arylamino, halogen, nitro, alkoxy,

and aryloxy radicals, X is selected from the group consisting of halogen, sulfo, p-toluenesulfo, and perchlorato radicals, R is selected from the group consisting of alkyl, aryl, halogen, nitro, amino, hydroxy, allcylamino, arylamino,

5 alkoxy and aryloxy radicals, R is selected from the group consisting of alkyl, and aryl-substituted radicals, n is a positive integer of the group consisting of 0 and l and m is a positive integer from 1 to 3, said photoconductive organic compound being employed in an amount of 1 part 10 per 0.3-2 parts by weight of film forming organic resin and in an amount of 5 to 40 g./ sq. meter of surface, the film forming organic resin having an electrical specific resistance of at leastlO ohms per centimeter which specific resistance is higher than the specific resistance of the support.

2. An electrophotographic material according to claim 1, wherein said photoconductive compound is a compound having the following formula:

x l CH oso orr Cl 3. An electrophotographic material according to claim a 1, wherein said photoconductive compound is a compound having the following formula:

having the following formula:.

n4; 11:0 H-NO 2 on osmorr 5. An electrophotographic material according to claim 1, wherein said photoconductive compound is a compound having the following formula:

6. An electrophotographic material according to claim 1, wherein said photoconductive compound is a compound having the following formula:

( 311:0 I-I-Q-NO 2 Cir 7 7. An electrophotographic material according to claim 1, wherein said photoconductive compound is a compound having the following formula:

R// R m wherein Y represents the radicals required to complete a ring system selected from the group consisting of (a) six-membered heterocyclic rings, (b) six-membered het erocyclic rings to which is fused a benzene or naphthalene ring, and (c) six-membered heterocyclic ring to which are fused a benzene or naphthalene ring containing at least one substituent selected from the group consisting of alkyl, aryl, amino, alkylamino, arylamino, halogen, nitro, alkoxy, and aryloxy radicals, X is selected from the group consisting of halogen, sulfo, p-toluenesulfo, and perchlorato radicals, R is selected from the group consisting of alkyl, aryl, halogen, nitro, amino, hydroxy, alkylamino, arylamino, alkoxy and aryloxy radicals R is selected from the group consisting of alkyl, and aryl-substituted radicals, n is a positive integer of the group consisting of 0 and 1 and m is a positive integer from 1 to 3, the photoconductive compound being employed in an amount of 1 part per 0.3 to 2 parts per weight of the film forming agent and in an amount of 5-40 g. per square meter of photoconductive layer.

9. A process according to claim 8 in which the said photoconductive compound has the following formula:

N \N\ on=on oi l on, oso om c1 10. A process according to claim 8 in which the said photoconductive compound has the following formula:

t o 11 05030 H3 12. A process according to claim 8 in which the said photoconductive compound has the following formula:

13. A process according to claim 8 in which the said photoconductive compound has the following formula:

ofi I 14. A process according to claim 8 in which the said photoconductive compound has the following formula:

W 01 ll -o H=C l b-QC 1 N I 1130 o SO3CH; or

said photoconductive being present in an amount of one part per 0.3-2 parts by weight of the binder and in an amount of 5-40 g./sq. meter of photoconductive layer.

References Cited by the Examiner UNITED STATES PATENTS 4/1949 White et al. 96l06 7/1959 Dawson et al. 96-114 

1. AN ELECTROPHOTOGRAPHIC MATERIAL COMPRISING A PHOTOCONDUCTING INSULATING LAYER CARRRIED ON A SUPPORT, SAID PHOTOCONDUCTING LAYER BEING COMPOSED OF A DIELECTRIC FILM FORMING ORGANIC RESIN AND A PHOTOCONDUCTIVE ORGANIC COMPOUND DISPERSED THEREIN, SAID PHOTOCONDUCTIVE ORGANIC COMPOUND HAVING THE FORMULA: 